The incident took place when the bus collided head-on with the autorickshaw that was carrying nine persons, Mahabubnagar (Rural) Circle Inspector Ramakrishna said.
Three occupants of the rickshaw were killed on the spot, while another passenger succumbed to the injuries during treatment, he said.
Three others who were injured are undergoing treatment, he said.
The driver of the bus has been taken into custody and investigation is underway, the police officer said.